1. Field of the Invention
The invention relates in general to a method for signal transmission and more particularly to a method of relaying messages in a mobile communication system.
2. Description of the Related Art
Mobile communication devices such as mobile phones are small and portable. People can communicate each other nearly anytime and anywhere. In addition, in the near future, notebook computers and personal digital assistants (PDAs) will be integrated into the mobile communication devices, so as to provide much more convenience.
FIG. 1 shows a conventional cellular mobile communication system 100. The service area of the cellular mobile communication system 100 is divided into a number of adjacent regions known as cells 101. The cells 101 are equipped with respective base stations (node-B) 102. Base stations 102 can communicated with user equipment unit (UE 103) when UE 103 stays in the service area of base station 102. UE 103 is assigned a dedicated identification (e.g. International Mobile Subscriber Identity, IMSI), and must submit the registration of the UE 103 for communication services, immediately after start-up, to a particular base station associated with the cell that the UE 103 is within. After that, the registered UE 103 can communicate through the base station only. In other words, the base stations 102 should know the updated information about the user equipment units of the subscribers staying in the service coverage of the associated base stations 102, and transmit messages to and receive messages from the user equipment units, such as the UE 103. The base stations 102 are connected to radio network controllers (RNC) 104(1) as shown in FIG. 1. In addition, a number of adjacent RNCs are connected to a mobile switching center (MSC) 104(2) (or a serving GPRS supported node (SGSN), wherein GPRS stands for general packet radio service). Through gateway mobile switching centers (GMSCs) 104 (or gateway GPRS supported nodes, GGSNs), the mobile communication network is connected to a public switched telephone network (PSTN) 106 in the local area. The PSTN 106 is a public communication network that combines fixed networks and radio networks.
FIG. 2 illustrates a conventional cellular mobile communication system and the communication procedure of UE 201 in the communication system. When UE 201 wants to send messages to another UE 201(1), the UE 201 transmits the messages by wireless link to a base station 203. The base station 203 transmits received messages to an RNC 204(1). The messages will be received by PSTN 206 through MSC 204(2) and GMSC 204(3). PSTN 206 will transfer the messages to a base station 203(1) via a GMSC 204(3), MSC 204(2), and RNC 204(1). The base station 203(1) will forward the received messages to UE 201(1) by wireless transmission. Likewise, according to the data transmission process, the UE 201 receives the message sent from the other UE, by the base station 203 of the cell 202 which the UE 201 is within.
In a wireless communication system, if the wireless transmission distance becomes longer, the signal distortion occurred in communication becomes more significant. If the coverage areas of some cells include barriers or protrusions, such as hills or tall buildings, the radio waves may be unable to penetrate these natural or man-made barriers. Besides, the radio waves may be reflected by the protrusions, and the original radio waves may be affected by destructive interference due to the reflected radio waves, thus resulting in signal distortion or noise interference to be more significant. Therefore, the wireless link between the UE 201 and the base station 203 might be affected by some barriers, and the quality of signals will be degraded and the strength of the received signals will even be dropped to be undetectable. More specifically, the carrier-to-interference ratio (C/I ratio) of each connection should be greater enough to support the required Frame Error Rate (FER).
By definition, the C/I ratio of a connection is proportional to the energy used per bit. The energy used per bit depends on the service rate and the transmission power of the connection. It means that we can increase the SIR of a connection by decreasing the service rate of a connection to combat with bad traffic environments. On the other hand, the service rate can be increased while under good traffic environments. In other words, the data rate for the UE 201 and base station 203 is related to the communication quality between UE 201 and the base station 203, wherein the communication quality is indicative of received signal power, interference from noise, and the degree of a distortion of signal.
FIG. 3A illustrates that UE 303 transmits a message to base station 302 in cell 301. If the coverage area of the cell 301 is too large, some portions of the cell 301 may be distant from the base station 302. If the UE 303 proceeds to communicate in such portions of the cell 301, data rate of data signals and voice communication quality would be affected. FIG. 3B illustrates that the UE 303 transmits a message to the base station 302, when a barrier, such as a hill or a tall building, exists in the cell 301. For example, a hill 304 exists between the UE 303 and the base station 302, as shown in FIG. 3B. Because the base station 302 and the UE 303 are separated by the hill 304, the received signals will be degraded in quality and the strength of the received signals will even be dropped to be undetectable.
In order to provide services with higher data rates, a conventional approach is to install additional base stations such that a cell is partitioned into a number of microcells. FIG. 4A illustrates a number of microcells 402 for wireless transmission, wherein microcells 402 are of the cell 401 and each of the microcells 402 has respective coverage area smaller than that of the cell 401. For a microcell that can communicate to a UE within the microcell's base station, the maximum allowable data rate from the UE 404 to the base station 403 of the microcell 402 is greater than that of the cell 401. If there is a barrier such as a hill 405 between the UE 404 and the cell 401, as illustrated in FIG. 4B, the communication between them would be hindered. If the approach that the cell 401 is subdivided into the microcells 402 is used, the influence of the barrier on the data transmission can be reduced to a minimum level.
Unfortunately, the conventional approach to dividing a cell into a number of smaller cells has the following disadvantages. First, the microcell may introduce additional costs due to the increase of base stations. Furthermore, it may not be applicable for rural and sub-urban areas due to the low subscriber density.